Computer system reliability is always a concern, but much more so in some settings than in others. In space, for example, maintenance, repair, and replacement of computer systems are either extremely expensive or impossible. Furthermore, failure of a computer system may lead to complete or partial mission failure. Environmental factors present in such settings exacerbate these concerns. In space, the environmental factors include both ionizing and non-ionizing radiation.
Computer equipment failures due to ionizing radiation are generally classified in three groups: (1) Total Ionizing Dose (“TID”), (2) Latch-up, and (3) Single Event Upsets (“SEUs”).
As the name implies, TID failures result from cumulative radiation exposure. TID failures can be dealt with by shielding computer equipment from radiation.
Single-event upsets result from change of state in an electronic device, such as a memory element. Thus, an SEU is not a “failure” in the conventional sense, but rather corruption of state information. Because heavy ions with high relative charges (i.e., multi-proton charges) tend to penetrate conventional shielding, such shielding generally does not reduce SEU occurrence frequency to an acceptable degree. The conventional approach to reducing SEUs due to heavy ions is to make radiation-hardened computer systems by designing custom cells for known computer architecture. This approach takes much time and money, and results in outdated designs. First, license for a known computer architecture (processor core) needs to be negotiated. Then, the cells of the processor core must be redesigned, and the processor core must be tested and put into production. The end result is a processor core that is older than then-current state-of-the-art commercial parts, and slower and less power-efficient than the original processor core architecture.